Mitosis and Meiosis

• An adult human contains an estimated 100 trillion cells and yet we start Life as a single cell
• Cell division in eukaryotic cells

  1. Interphase

  2. Prophase

  3. Metaphase

  4. Anaphase

  5. Telophase
• Interphase


  Can be divided into three distinct phases: G1, S, and G2
• Interphase


  1. G1 phase: Organelles and cytoplasmic components replicate

  2. S phase: DNA replicates

  3. G2 phase: Enzymes needed for cell division are produced
• Most eukaryotic cells spend a great deal of time in interphase and a very short period of time actually dividing
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase


  • Chromosomes condense and become visible as two sister chromatids
  • Cytoskeleton disassembles as the spindle begins to form
  • Centrioles migrate to opposite poles
  • Nuclear envelope breaks down
• Metaphase


  • Chromosomes align at the equator of the cell (metaphase plate)
• Anaphase


  1. Proteins holding sister chromatids together degrade, freeing individual chromosomes

  2. Chromosomes are pulled by their kinetochores to opposite poles
• Telophase


  1. Cleavage furrow forms in the center of the cell

  2. Chromosomes decondense as the nuclear envelope reforms

  3. Spindle apparatus disassembles
• Cytokinesis


  Completes cell division by extending the cleavage furrow to completely separate the daughter cells
• Cell cycle checkpoints


  • Ensure the process is occurring accurately
  • If not, the cell cycle will stop at the checkpoint and correct or inhibit cell division
• Factors regulating cell cycle


  • Growth factors
  • Cell size
  • Nutritional state
• The entire process of cell division can take on average 10 to 20 hours in a typical plant or animal cell
• Meiosis generates the gametes for reproduction
• Meiosis


  The process by which haploid cells are produced from a diploid cell
• Meiosis


  1. Meiosis I

  2. Meiosis II
• Meiosis I


  • Homologous pairs of sister chromatids lie side by side (synapsis)
  • Crossing over occurs between non-sister chromatids
• Meiosis II


  • No additional DNA replication
  • Sister chromatids separate and move to opposite poles
• Meiosis results in four haploid daughter cells, while mitosis results in two diploid daughter cells
• Genetic diversity in meiosis

  • Crossing over
  • Independent assortment of homologous chromosomes
  • Random pairing of gametes during fertilization
• The combination of independent assortment, crossing over, and random pairing of gametes increases genetic diversity and explains why a child will not look exactly like their parents or siblings